1. Field of the Invention
The present invention relates to a semiconductor laser device and particularly to a laser device for use in an optical memory or the like, which emits a plurality of beams.
2. Description of the Prior Art
Japanese Utility Model Laying-Open Gazette No. 152474/1987 describes that a semiconductor laser device for emitting laser beams along a plurality of parallel axes (i.e., a multibeam laser device) is suitably used as a light source for an optical memory or the like. For example, a multibeam laser device having three parallel laser resonators, the outputs of which can be controlled independently of one another, is able to record, reproduce and erase data on an optical memory by using three beams emitted from the respective laser resonators. Thus, it becomes possible to detect recording errors in real time, to control recording conditions and to simultaneously record and reproduce data.
Generally, since output characteristics of a semiconductor laser are very sensitive to environmental conditions such as changes in temperature, one of the two beams emitted in opposite directions along an axis from both ends of a semiconductor laser resonator is used for application of a signal and the other beam is used for monitoring of an optical output of the laser resonator (as disclosed for example in Japanese Patent Laying-Open Gazette No. 102590/1983). In the monitoring, light intensity of the beam is measured by a photodetector (e.g., a PIN diode) intersecting with the beam path. Thus, electric power supplied to the laser resonator is controlled based on the monitoring result so that intensity of the laser beam used for the signal becomes a predetermined value.
FIG. 1 is a schematic illustration of a three-beam laser device of the prior art used in an optical pickup for an optical memory. A semiconductor laser 1 for emitting beams along three axes is set on a heat sink 4 of silicon fixed to an L-shaped block 5 of copper and the like. The semiconductor laser 1 includes three laser resonators, the outputs of which can be controlled independently of one another, and it emits forward beams 11a, 12a and 13a and backward beams 11b, 12b and 13b along the three parallel axes. A photodetector 2 is set on the L-shaped block 5 so as to receive the backward beams 11b, 12b and 13b. The photodetector 2 includes photodetector elements 21, 22 and 23 for monitoring intensities of the backward beams 11b, 12b and 13b, respectively. The block 5 is fixed on a stem 8 and is closed in a cover cap 7 having a window 6. The forward beams 11a, 12a and 13a are focused on an optical memory medium (not shown) by a lens system (not shown) through the window 6. distance between the respective adjacent beam axes need to be considerably small, i.e., in a range of 70 to 100 .mu.m in making account of a recording density.
FIG. 2 is a schematic illustration showing geometrical relations of the backward beams 11b, 12b and 13b to the photodetector elements 21, 22 and 23. In FIG. 2, each of the distances between the adjacent beam axes 11, 12 and 13 is 100 .mu.m and the photodetector elements 21, 22 and 23 intersect with the beam axes 11, 12 and 13, respectively. As shown in this figure, if the distance between the semiconductor laser 1 and the photodetector 2 is large, the photodetector elements 21, 22 and 23 receive not only the corresponding beams 11b, 12b and 13b but also outer parts of the adjacent diverged beams. Therefore, the outputs of the three laser resonators cannot be accurately monitored independently of one another.
In the case of the divergence angle .theta. of each beam being 10.degree., overlap between the adjacent diverged beams does not occur on the photodetector 2 if the distance between the semiconductor laser 1 and the photodetector 2 is less than 500 .mu.m. In that case, however, each of the intervals of light receiving areas of the photodetector elements 21, 22 and 23 need to be 10 to 30 .mu.m. Accordingly, if the photodetector is of an integration type, electric current of about 0.1 to 2% leaks between the adjacent photodetector elements, causing crosstalk. Thus, the outputs of the three laser resonators cannot be accurately monitored independently.